Match List I with List II
List I                                   List II
 Coordination entity           Wavelength of light absorbed in nm
A. [CoCl(NH$$_3$$)$$_5$$]$$^{2+}$$               I. 310
B. [Co(NH$$_3$$)$$_6$$]$$^{3+}$$                 II. 475
C. [Co(CN)$$_6$$]$$^{3-}$$                   III. 535
D. [Cu(H$$_2$$O)$$_4$$]$$^{2+}$$                 IV. 600
Choose the correct answer from the options given below :-

To determine the correct matching between the coordination entities and the wavelengths of light they absorb, we use Crystal Field Theory (CFT).

The wavelength of light absorbed $$\lambda$$ is inversely proportional to the crystal field splitting energy $$\Delta_o$$:

$$E=\frac{hc}{\lambda}\implies \Delta_o\propto\frac{1}{\lambda}$$

Hence, stronger field ligands produce a larger crystal field splitting and absorb light of shorter wavelength.

According to the spectrochemical series,

$$CN^- > NH_3 > H_2O > Cl^-$$

Evaluating the coordination entities

• $$[Co(CN)_6]^{3-}$$
  • $$CN^-$$ is the strongest field ligand among the given ligands.
  • It produces the largest value of $$\Delta_o$$ and therefore absorbs the shortest wavelength.
  • Match: I (310 nm)
• $$[Co(NH_3)_6]^{3+}$$
  • $$NH_3$$ is a strong field ligand but weaker than $$CN^-$$.
  • Hence, it absorbs a longer wavelength than $$[Co(CN)_6]^{3-}$$.
  • Match: II (475 nm)
• $$[CoCl(NH_3)_5]^{2+}$$
  • Replacing one $$NH_3$$ ligand by the weaker field ligand $$Cl^-$$ decreases the crystal field splitting.
  • Therefore, the absorbed wavelength increases compared to $$[Co(NH_3)_6]^{3+}$$.
  • Match: III (535 nm)
• $$[Cu(H_2O)_4]^{2+}$$
  • $$H_2O$$ is a weaker field ligand than $$NH_3$$ and $$CN^-$$.
  • Hence, it produces a smaller value of $$\Delta_o$$ and absorbs the longest wavelength among the given complexes.
  • Match: IV (600 nm)

Therefore, the correct matching is

• A → III
• B → II
• C → I
• D → IV

which corresponds to

$$\boxed{\text{A-III,; B-II,; C-I,; D-IV}}$$